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\begin{document}

\title{Masks for the public: laying straw men to rest}



\author[1]{Trisha Greenhalgh}%
\affil[1]{University of Oxford}%


\vspace{-1em}



  
  \date{March 13, 2024}


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\selectlanguage{english}
\begin{abstract}
This paper responds to one by Graham Martin and colleagues, who offered
a critique of my previous publications on masks for the lay public in
the Covid-19 pandemic. I address their charges that my co-authors and I
had misapplied the precautionary principle; drawn conclusions that were
not supported by empirical research; and failed to take account of
potential harms. But before that, I remind Martin et al that the
evidence on mask wearing goes beyond the contested trials and
observational studies they place centre stage. I set out some key
findings from basic science, epidemiology, mathematical modelling, case
studies and natural experiments, and use this rich and diverse body of
evidence as the backdrop for my rebuttal of their narrowly-framed
objections. I challenge my critics' apparent assumption that a
particular kind of systematic review should be valorised over narrative
and real-world evidence, since stories are crucial to both our
scientific understanding and our moral imagination. I conclude by
thanking my academic adversaries for the intellectual sparring match,
but exhort them to remember our professional accountability to a society
in crisis. It is time to lay straw men to rest and engage,
scientifically and morally, with the dreadful tragedy that is unfolding
across the world.%
\end{abstract}%



\sloppy


\par\null

\subsection*{Introduction}

{\label{introduction}}

Since the Covid-19 pandemic emerged, I have co-authored several articles
in both the academic and lay literature supporting the wearing of face
masks by the general public.\textsuperscript{1-3} In response to
negative criticism on social media, I put out a challenge: either write
a point by point critique of my papers or back off. Graham Martin and
colleagues responded with a preprint paper\textsuperscript{4} and a
rapid BMJ response.\textsuperscript{5}

Later in this article, I will address their substantive scientific
points, which I will number and put in italics. I have tried to
represent their points faithfully, without exaggeration, and apologise
in advance if I have not captured their intended nuances. But first, let
me highlight a subtle rhetorical move by Martin et al: \emph{they
completely ignore various types of evidence} -- including basic science,
mathematical modelling and real-world case examples of asymptomatic
transmission and super-spreader events. Before addressing what they did
talk about in their paper,\textsuperscript{4} I set out some important
scientific evidence that they did \emph{not} talk about. I draw heavily
on the primary sources cited in the narrative review by Howard et
al.\textsuperscript{6}

\subsection*{A wider evidence base}

{\label{a-wider-evidence-base}}

The basic science of Covid-19 is important. The Sars-CoV-2 virus which
causes this disease replicates in the upper respiratory tract (in
contrast to the causative agent of Sars-CoV-1, which is a less
contagious lower respiratory tract virus).\textsuperscript{7 8} This
means it is likely to be transmitted mainly by droplets (which is why
there is so much emphasis on hand-washing, since droplets contaminate
surfaces). Droplets emitted from the human respiratory tract (which are
relatively large) quickly turn into aerosols (smaller
micro-droplets),\textsuperscript{9} so unless they are controlled at
source, they become harder to block.

A crucially important point, which is often overlooked by doctors,
systematic reviewers and the lay press (and which was not addressed at
all by Martin et al), is that most research on masks -- almost all of
which has been undertaken in the context of healthcare workers --
considers the extent to which they \emph{protect the wearer} . The
current question we need to address is a different one: whether mask
wearing \emph{protects other people from droplets emitted by the
wearer}-- a measure known as source control. Source control works in a
different way to wearer-protection -- by blocking large droplets as they
are emitted in coughing, sneezing and talking and before they become
aerosolised.\textsuperscript{10-12} Large droplets (and indeed a
proportion of aerosols) are blocked -- not perfectly, but significantly
-- by cotton home-made masks.\textsuperscript{13-16}

Masks that protect the wearer work by blocking tiny aerosolised
particles. For this reason, medical-grade masks need to meet stringent
filtration standards, about which much has been written (see for
example\textsuperscript{17}). In contrast, source control masks can
potentially be very effective even if they only block the larger droplet
particles. Studies of the efficacy of masks in protecting the wearer are
therefore irrelevant to the question of source control.

Evidence of asymptomatic carriage of Sars-CoV-2 is strong and
consistent. Oran and Topol have analysed (to date) 12 such examples from
around the world,\textsuperscript{18} including cohorts identified for
nationwide testing (Iceland), local population testing (Vo, Italy),
passengers or crew of three ships (Diamond Princess, USS Theodore
Roosevelt and Charles de Gaulle aircraft carrier), nursing home staff
and residents (USA), residents of two homeless shelters (Boston and Los
Angeles), ex-pats (Japanese evacuated from Wuhan and Greek citizens
evacuated from other countries), and pregnant women (New York City
obstetric patients). In these diverse cohorts, between 31\% and 88\% of
positive cases were asymptomatic or pre-symptomatic when tested. A
recent editorial in the New England Journal of Medicine argued that the
exceptionally high rates of asymptomatic transmission of Sars-CoV-2 call
for a different approach to infection control -- specifically, masks for
the public.\textsuperscript{19}

In contrast to the high transmission rates from such individuals in this
case series, there are some impressive case examples of infected
individuals \emph{not} passing on the virus when wearing a mask. For
example one man flew from China to Toronto wearing a mask for the entire
flight, became symptomatic the next day and tested positive for
covid-19; none of the other passengers or crew became
infected.\textsuperscript{20}

Another piece of evidence that masks could make a big difference is
super-spreader events, a list of which has been compiled by
Kay.\textsuperscript{18} Perhaps the most dramatic is the choir practice
in Seattle, in which, despite maintaining a degree of social distancing
during the rehearsal, 45 of 60 people became infected and two (so far)
have died.\textsuperscript{21} In all these super-spreader events,
extensive transmission was traced back to close contact -- but not
necessarily physical touching. As the authors put it: \emph{``When do
COVID-19 {[}super-spreader events{]} happen? \ldots{} Wherever and
whenever people are up in each other's faces, laughing, shouting,
cheering, sobbing, singing, greeting, and praying.''}

In relation to a community-wide intervention such as mask-wearing, we do
not need to prevent every transmission of every droplet or every viral
particle. As with handwashing and social distancing, the objective of
the policy is more modest: to achieve a \emph{substantial reduction} in
the transmission rate of the virus. Every infectious disease has a
transmission rate (R0). A disease with an R0 of 1.0 means that each
infected person, on average, infects one other person. A disease whose
R0 is less than 1.0 will die out. The strain of flu that caused the 1918
pandemic had an R0 of 1.8. The R0 of Sars-CoV-2 was estimated at 2.4 by
Imperial College researchers,\textsuperscript{22} and other research
suggests it could even higher.\textsuperscript{23} A population measure
that reduces the transmission rate (``effective R0'' or
R\textsubscript{eff}) to below 1.0 will be highly effective, \emph{even
if some cases of transmission still occur} .

Mathematical modelling suggests that a mask that is 60\% effective at
blocking viral transmission and is worn by 60\% of the population will
reduce R0 to below 1.0.\textsuperscript{24} This leaves plenty of room
for error as people make their own imperfect masks from old clothing and
as some people either cannot or will not wear a mask. Not all
respiratory viruses are filtered equally by face masks; masks appear to
be more efficient at blocking Sars-Cov-2 than rhinoviruses, for
example.\textsuperscript{25} Materials scientists have shown that whilst
different fabrics are more or less efficient at blocking particle
transmission, cotton weaves with high thread count or a double layer of
two different fabrics (e.g. cotton-flannel) typically provides high
filtration efficiency.\textsuperscript{26}

There are now many natural experiments of mask-wearing in Covid-19, as
countries introduce either mandatory or voluntary mask-wearing policies.
Of note is the example of the Czech Republic and Austria, both of which
introduced social distancing on the same day; the former also introduced
compulsory mask wearing. New covid-19 infections fell more quickly in
the Czech Republic, and only began to fall in Austria after masks were
made mandatory two weeks later.\textsuperscript{3} Also noteworthy is
the observation that every single country where mask-wearing has been
introduced as national policy (often but not always alongside other
measures), rates of transmission fell in the subsequent days.

All these various streams of evidence contribute, in different ways and
at different levels, to strengthen the argument for mask wearing. With
this wider evidentiary context sketched, let me now take on the specific
claims made by Martin et al in their paper and rapid
response:\textsuperscript{4 5}

\subsection*{\texorpdfstring{Precautionary principle, ``weak'' evidence
and potential
harms}{Precautionary principle, weak evidence and potential harms}}

{\label{precautionary-principle-weak-evidence-and-potential-harms}}

\emph{The precautionary principle we invoked to justify wearing of
masks\textsuperscript{1} is {[}Martin et al imply{]} irrelevant, because
it is normally used to advise caution in the uptake of innovations with
known benefits but uncertain or unmeasurable downsides, such as exposure
of the public to radiation.}

The term ``precautionary principle'' does not have a fixed meaning,
though I accept that it is more usually invoked as described by Martin
et al. It may surely prove equally appropriate a) when harm is not
currently happening but a proposed intervention may cause harm and b)
when serious harm is currently happening and a proposed intervention may
reduce that harm. There seems to me to be a strong symmetry between
these examples. One does not cancel the other out. Both the omission in
the former case and the act in the latter case are measures aimed at
preventing harm.

\emph{``{[}T{]}he very weak evidence for face masks should be
reiterated''. Trials have shown no evidence of reduced transmission with
masks compared to no masks, and observational studies are contaminated
with multiple confounders (e.g. parallel introduction of other measures
such as hand washing).}

The evidence base for face masks (described above) is not weak. However,
it was a weak rhetorical move for Martin et al to ignore the strongest
evidence when penning their critique. Our BMJ analysis article briefly
reviewed the literature from experimental trials and systematic
reviews.\textsuperscript{1} Two pre-print systematic
reviews\textsuperscript{27 28} and a narrative
review\textsuperscript{6}were all published the same week. In all those
syntheses, there is a conspicuous \emph{absence of experimental
evidence} in relation to the wearing of masks \emph{in public places,}
\emph{by the lay public,} as source control to \emph{prevent community
transmission}of any respiratory illness.

The sum total of randomised trials and observational studies covered in
these reviews, \emph{all of which are irrelevant to the question of
source control} , comprise: a) studies of mask-wearing within the home
to reduce contagion to other family members;\textsuperscript{29-38} b)
studies of occupational exposure (e.g. workers in poultry
factories);\textsuperscript{39 40} c) studies of specific mass events
(notably, pilgrimages to the Hajj);\textsuperscript{41-47} d) studies in
schools and university halls of residence;\textsuperscript{48-51} e)
studies of air travel;\textsuperscript{52} f) studies of healthcare
workers;\textsuperscript{38 53-55} and just two studies of general
community prevention: an attempt to prevent the common cold in
Finland,\textsuperscript{56} and a paper on behavioural measures (among
other things) in the prevention of SARS, in which those who ``always''
wore a mask when outside the home had a relative risk of developing the
disease of 0.3 compared to those who ``never'' wore
one.\textsuperscript{57}

All of these primary studies were designed to test the hypothesis that
wearing a mask in the specific situation described in (a) to (g)
above\emph{protects the wearer} . The question my colleagues and I have
addressed in our articles\textsuperscript{1-3} was a completely
different one: whether mask wearing by a member of the general
public\emph{protects others in the community} . Martin et al's depiction
of the evidence from trials and observational cohort studies as ``very
weak'' is incorrect. Such randomised controlled trial evidence, in
relation to source control, is \emph{entirely absent} and unrelated
evidence should not be presented as a possible answer. (Note: this does
not mean there is no evidence at all -- merely, that there is no
evidence valued by the RCT community).

Absence of trial evidence is partly due to the fact that experimental
studies of mass public health measures are usually impractical. We don't
randomise schools to close, towns to go into lockdown, people to sneeze
into their elbows or whole communities to wash their hands regularly.
That is simply not how mass public health interventions get tested. The
argument that we should not recommend masks \emph{because there are no
published experiments} is out of step with other public health policy on
infection control in general and covid-19 in particular. As with other
public health measures, we should make a decision based on an assessment
of the full body of evidence described above.

\emph{Wearing face masks may cause harm, specifically {[}citing the
Jefferson systematic review\textsuperscript{28}{]} ``discomfort,
dehydration, facial dermatitis, distress, headaches, exhaustion''.}

It is widely reported that prolonged use of personal protective
equipment by healthcare personnel in pandemic contexts is associated
with all the problems listed (though exhaustion in particular may have
other explanations in such circumstances). Some research studies have
confirmed that prolonged wearing of medical-grade masks by healthcare
workers can result in physical and psychological
harms.\textsuperscript{58-61} However, neither Martin et al nor the
Jefferson systematic review which they cite offer any evidence
whatsoever that the use of home-made cloth masks by the lay public for
source control has been shown to cause such harms. Indeed, there is no
common-sense reason why a mask made out of one's own old t-shirt would
cause illness when the t-shirt itself was well tolerated (and if it
wasn't, why make a mask out of it?). The possible irritant effect of a
mask should also be weighed against its potential benefit.

\subsection*{Can the general public be
trusted?}

{\label{can-the-general-public-be-trusted}}

\emph{The general public are unlikely to use masks ``properly''. Even
healthcare workers struggle to achieve necessary standards of donning
and doffing technique, and ``inappropriately discarded masks present an
infection risk''.}

Infection control standards designed for healthcare workers are
irrelevant to the general public. The infected particles on a healthcare
worker's mask are likely to come from patients, whereas the mask wearer
is (hopefully) uninfected and therefore vulnerable. In contrast, if a
member of the public is wearing a cloth mask, they are most likely
source of any infection on their own mask. The more infectious particles
that are caught in that mask, the fewer will have been aerosolised to
infect others. A mask that has been removed does not need to be
disinfected, and formal doffing is not needed (though hand-washing would
be sensible in case the mask is contaminated with droplets from others).
Sars-CoV-2 has a lipid membrane which is destroyed by soap or detergent
(this, of course, is why hand-washing works). A cloth mask can be
laundered along with other clothing in a normal hot
wash.\textsuperscript{62} An alternative option in low-income countries
is to wash the mask with soap and water and leave it to dry in the sun.
Imposing unnecessarily high standards of disinfection on the public is
likely to reduce the uptake of the measure and be counterproductive.

\emph{Being able to make, don, doff and disinfect your own cloth mask is
a middle-class privilege. The efficacy of masks in the general
population will be reduced by ``the potential for great variation in
materials, fit, adherence, touching and adjustment, doffing, disposal,
frequency of laundering and so on''.}

There is no need to standardise the design of masks or fetishise how
they are worn, any more than we do so for shoes to protect our feet.
Cotton and similar materials do not block droplets entirely -- but most
double-layer fabrics seem to filter more than 90\% of
them.\textsuperscript{13-16} As noted above, if 60\% of people wear a
mask that is 60\% effective, this is likely to be sufficient to
substantially reduce the transmission of Sars-CoV-2. To say that because
some people may find it difficult to obtain or launder a mask, we should
not recommend them for anyone is illogical -- especially since adverse
socio-economic circumstances is a risk factor for developing Covid-19
and also for poorer prognosis.\textsuperscript{63} The negative,
individualist emphasis of Martin et al's critique ignores the positive
impact of mask-making initiatives as a component of wider community
resilience strategies in Covid-19.\textsuperscript{64} The South African
Government, for example, has recently issued a tender for community
sewing co-operatives to supply cloth face masks.\textsuperscript{65}

\emph{Risk compensation (in which people made to wear masks reduced
other infection control behaviours such as hand-washing) could occur}

Martin et al cite a review from 20 years ago which describes mixed
findings on risk compensation behaviours.\textsuperscript{66} They fail
to cite a more recent review suggesting that such behaviours appear
rare.\textsuperscript{67} Both these reviews, however, focused mainly on
injury prevention, not on infection control measures. More relevant
perhaps are studies showing that teenagers vaccinated against human
papilloma virus do not appear to take more sexual
risks,\textsuperscript{68 69} though there is some evidence that
pre-exposure prophylaxis may increase sexual risk-taking in men who have
sex with men.\textsuperscript{70} The argument that risk compensation
behaviour would occur specifically in relation to masks in the context
of Covid-19 is entirely speculative. It is also unlikely. If adverse
behaviour change happens to a significant degree, we would surely have
seen some examples from around the world by now, since dozens of
countries have now made mask-wearing mandatory.

\subsection*{Unintended consequences?}

{\label{unintended-consequences}}

\emph{``{[}U{]}niversal mask-wearing might aggravate the climate of fear
already documented for Covid-19''}

Fear is perhaps a reasonable response to a deadly pandemic that has so
far affected at least three million people and cost hundreds of
thousands of lives. There is no evidence whatsoever that mask-wearing
policies increase fear. The counter-argument -- that such a measure
would help \emph{reduce} fear -- is equally plausible (though there is
no actual evidence either way). In studies of community mask use in
tuberculosis control, mask-wearing by affected individuals reduced
disease transmission but increased stigma,\textsuperscript{71 72}whereas
promotion of mask-wearing by all members of the community was associated
with destigmatisation.\textsuperscript{72} The relevance of these
findings to the current pandemic are unclear.

\emph{Promoting mask-wearing by the lay public could lead to a shortage
of medical-grade masks.}

This is a real concern, but it is not a reason to distort or deny the
evidence of benefit. There is no reason why the public should wear
medical-grade masks, since cotton masks are more comfortable, recyclable
and sufficiently effective for source control. I accept that a public
information campaign would be needed to get this message across to lay
people as well as to clinicians and scientists (most of whom, like
Martin et al, have unjustifiable extrapolated findings from research on
infection control in healthcare settings and sought to apply the same
standards to the public). In any case, simple surgical masks could be
produced easily and in large numbers by repurposing manufacturing
capacity if the political will was there.\textsuperscript{73}

\emph{``{[}B{]}usinesses or states might see widespread or mandatory
mask-wearing as a warrant for a premature return to `business as usual',
justifying unsafe workplaces or crowded commuting conditions in terms of
the protection offered by masks.''}

This statement is entirely speculative. No evidence is given for it and
it implies that the preferred state is for society to remain in lockdown
indefinitely. The risks to the economy of prolonged lockdown are
dire.\textsuperscript{74 75} Recession and job losses will have a
disproportionate effect on the poor and socially excluded. There are
ethical as well as scientific arguments for considering all measures
that may help to reduce the lockdown period and get businesses up and
running as a matter of urgency.

\emph{Masks are an example of a complex intervention in a complex
system. Their effects are impossible to predict, therefore we should not
introduce them.}

The papers cited to support this assertion (one of which was co-authored
by me\textsuperscript{76}) actually support the opposite conclusion.
Just because a complex system is unpredictable does not mean we should
do nothing.\textsuperscript{77} As Martin et al acknowledge, careful
data collection and frequent, timely analysis that feeds into adjustment
of policy will allow an adaptive and data-driven response. Their
depiction of current UK policy as too ``blunt'' to respond in this way
is conflating politics with science. It is not an argument to sit idle
when hundreds are dying daily.

\subsection*{\texorpdfstring{``Systematic'' versus narrative
reviews}{Systematic versus narrative reviews}}

{\label{systematic-versus-narrative-reviews}}

In the first paragraph of their paper, Martin et al contrast ``two
{[}preprint{]} systematic reviews'' with ``another preprint review, with
more opaque methods but encompassing an eclectic range of disciplinary
perspectives''. The implication is that the conclusions of
``systematic'' reviews which favour controlled experiments are
necessarily more reliable than those of ``opaque'' and ``eclectic''
narrative reviews which bring in so-called anecdotal evidence and
arguments from basic and social sciences. Elsewhere, colleagues and I
have challenged this conceptual bias.\textsuperscript{78} In that paper,
we distinguish between narrowly-defined biomedical questions that can be
answered using conventional systematic review, with meta-analysis where
appropriate, and more complex, multifaceted problems that
require\emph{clarification and insight,} for which a more interpretive
and discursive synthesis of is needed.

Looking back at the first part of this paper, where I summarised the
evidence that Martin et al chose to ignore, I am struck by how they
dismissed the \emph{stories} (the Covid-stricken choir, the air
passenger whose mask may have saved a plane-load of people from
contagion, the cruise ships that became floating quarantine prisons).
But these stories are crucial to both our scientific understanding and
our moral imagination. Their contrasting plots -- tragedy, melodrama,
lucky escape -- pull together complex chains of influence and remind us
that causality in a pandemic is rarely linear. Anecdote may be a low
form of evidence in some taxonomies, but each one calls for an
explanation.

As my co-authors and I concluded in out article on narrative review:

\emph{``Training in systematic reviews has produced a generation of
scholars who are skilled in the technical tasks of searching, sorting,
checking against inclusion criteria, tabulating extracted data and
generating `grand means' and confidence intervals. These skills are
important, but \ldots{} critics may incorrectly assume that they
override and make redundant the generation of understanding. \ldots{}
While there are occasions when systematic review is the ideal approach
to answering specific forms of questions, the absence of thoughtful,
interpretive critical reflection can render such products hollow,
misleading and potentially harmful.''} \textsuperscript{78}

\subsection*{Conclusion}

{\label{conclusion}}

In conclusion, I congratulate Martin et al for rising to my challenge to
produce a critique of my publications on face masks for the public. But
whilst academic sparring can keep a few sociologists amused during
lockdown, we also need to remember our moral accountability to a society
in crisis. I myself trained as a doctor. The relentless, day on day
stories of avoidable deaths from this dreadful disease sicken me. I will
do whatever I can, as an academic, a doctor and a citizen, to reduce
that death toll and help get society back running again.

As Gandhi et al concluded in their NEJM editorial: ``This unprecedented
pandemic calls for unprecedented measures to achieve its ultimate
defeat''.\textsuperscript{19} It is time to put the straw men to rest
and take a less mischievous perspective on the evidence.

\subsection*{Acknowledgements}

{\label{acknowledgements}}

\emph{Thanks to Helene-Mari van der Westhuizen for helpful comments on
an earlier draft, and to co-authors of my earlier papers (Manuel Schmid,
Thomas Czypionka, Dirk Bassler,} \emph{Laurence Gruer, Jeremy Howard,
Zeynep Tufekci) and Babak Javid for ideas and inspiration. I acknowledge
funding from the Wellcome Trust (WT104830MA) and UK National Institute
for Health Research (BRC-1215-20008). Responsibility for this article is
mine alone.}

\subsection*{}

{\label{section-1}}

\subsection*{}

{\label{section-2}}

\subsection*{}

{\label{section-3}}

\subsection*{References}

{\label{references}}

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